1. Field of the Invention
The present invention relates to a zoom optical system which can be used in various devices such as an image-pickup apparatus, a projector, an aligner, and a reader.
2. Description of the Related Art
Recently, along with the wide spread of digital cameras and mobile phones equipped with cameras, a compact and high-resolution zoom optical system has been demanded.
In a compact and high-resolution zoom optical system, the system is generally zoomed by moving a plurality of lens groups relative to a light-receiving plane (such as a CCD) in the optical axial direction. In a conventional zoom system in which lens groups are moved toward an object, the entire optical length (the distance between a first lens surface and an image plane) is increased, restricting the entire lens system from being miniaturized.
Whereas, an optical system has been discussed using an optical device called an Alvarez lens, the power of the entire system is changed by moving the optical device in a direction different from the optical axis (U.S. Pat. No. 3,305,294, U.S. Pat. No. 3,583,790, and “KOGAKU (Japanese Journal of Optics)” Vol. 29, No. 3 (2000)).
Various zoom optical systems are discussed for zooming with the Alvarez lens (Japanese Patent Laid-Open No. H02-119103).
In the optical system discussed in U.S. Pat. No. 3,305,294, the power is changed by relatively moving two lenses with curved surfaces expressed by a cubic function in a direction different from the optical axial direction. Since this optical system is not brought out in the optical axial direction, the entire lens length is reduced when used in a zoom optical system.
In the optical system discussed in U.S. Pat. No. 3,583,790, aberration is reduced by forming a lens in a curved surface expressed not only by the cubic function but also by the higher-order, for example the fifth-order function.
Furthermore, Japanese Patent Laid-Open No. H02-119103 discusses an example using this lens in a zoom optical system, and a principle that the power is changed by arranging at least two of the above lenses while the image point being maintained constant.
On the other hand, “KOGAKU (Japanese Journal of Optics)” Vol. 29, No. 3 (2000) describes an optical system including a rotationally asymmetrical optical device. This optical system has not a common axis (optical axis) differently from a general coaxial lens system. Such a non-coaxial optical system is called as an off-axial optical system that can be defined as an optical system including a curved surface (off-axial curved surface), in which when the optical path of the light passing through the image point and the pupil center is assumed as a reference axis, a normal of the surface at the intersecting point of the curved surface and the reference axis is not aligned with the reference axis.
Since the reference axis is bent in this case, the paraxial amount calculation has to use not a paraxial theory for a coaxial system but a paraxial theory based on an off-axial principle. The method is described in “KOGAKU (Japanese Journal of Optics)” Vol. 29, No. 3 (2000) in detail, which uses a 4×4 determinant on the basis of the radius of curvature of the surface, for example.
In U.S. Pat. Nos. 3,305,294 and 3,583,790, a method is described for changing the power and correcting the aberration using a pair of rotationally asymmetric lenses; however, the image plane cannot be maintained constant when the power is changed.
In Japanese Patent Laid-Open No. H02-119103, a principle is described for changing the power while maintaining the image pane constant; however, it has not yet led to a practical design for obtaining excellent images by correcting or reducing the aberration.
For configuring a zoom optical system using the Alvarez lens, it is useful that the image plane be maintained constant during zooming wile the change in aberration due to the zooming be reduced.